Switches in analog and radiofrequency (RF) applications often must deal with a wide dynamic range of signal strength. Transmitters, in particular, sometimes have to handle very high peak voltages. This can be a problem in the field of switch design, as the signal strength may exceed the voltage breakdown of the device. Another problem is that the control voltages available are much smaller than the signal strength. This makes it difficult to keep the switches in an on or off position. Switches for wireless handsets are a notable example of a system exhibiting these problems. In a GSM handset, for instance, the maximum signal strength may be as high as 35 dBm. Transmission through a 50 Ohm system results in a peak voltage of 17.88 V, while the control voltage and maximum available supply voltage are 2.5 V and 3.5 V, respectively.
FIG. 1 demonstrates one of the oldest circuits used to deal with this problem. In this case discrete PIN diodes are used as the switching element. This type of diode exhibits excellent RF characteristics with a large breakdown voltage. Direct current (DC) voltages are used to forward or reverse bias the diodes for a low or high impedance. A quarter wave matching network is required to isolate the off port from the on port. This solution works well in multi-port systems. However, high performance PIN diodes are not easily integrated. Further, a large number of passive elements are required to provide the bias and matching. Another significant problem is the current necessary to forward bias the diodes. This may be acceptable in a simple transmit-receive system, as the design may be configured so that the on diode is only used in the transmit mode. Multi-port systems, however, require current in receive mode as well.
Another common solution is shown in FIG. 2. In this case, field effect transistors (FETs) are used for the switching elements. Gallium arsenide (GaAs) pseudomorphic high electron mobility transistors (PHEMTs) are most commonly used due to their low loss and high breakdown voltage. However, the breakdown voltage is only about 16 V, which is too low to handle high signal level of a GSM system by itself. Also, a control voltage of 2.5 V will result in the off transistor turning on during the negative swing of the output signal. The solution to these problems is to use multiple FETs in series, as shown in FIG. 3. This effectively divides the signal voltage evenly across each transistor. This solution is capable of handling the high signal levels while introducing an acceptable amount of loss. It also has the advantage of a near zero current requirement, may be configured for multiple ports applications, and may be integrated on a single die. A disadvantage is that a large number of control signals are required. The lack of a complementary transistor technology on GaAs means that any logic functions will draw significant amounts of current. A separate CMOS control chip is often used with the GaAs switch die for this reason. Also, the use of an exotic technology means that the switch cannot be integrated with the other functions in the handset.
Attempts have been made to use CMOS as a switching technology with limited success. In some cases, a DC converter has been used to overcome the control signal limitations. However, the high loss of the substrate has been unacceptable. Silicon-on-Sapphire (SOS) and other exotic technologies have overcome this problem, but the high cost makes then unsuitable for integration with other functions.